Acute liver failure is a disease with a mortality of 60-90% depending on the cause, despite maximal supportive intensive care treatment. Even though liver transplantation remains the main curative option for patients suffering from acute liver failure, it is constrained by cost, donor scarcity and donor compatibility. A high incidence of patients with acute liver failure is dying on the transplantation waiting list. Artificial liver assist devices are particularly useful to sustain patients with acute and chronic liver failure that are awaiting liver transplantation or recovery/regeneration of their liver, but are ideally also capable of improving a patient's condition and converting patients from a non-transplantable to a transplantable state. Such devices ideally have to fulfil a variety of functions such as detoxification, regulation and synthesis of molecules in a manner resembling a healthy liver.
Devices for artificially providing liver functions can be grossly divided into non-biological and biological liver support. Early attempts of treating liver failure focused on non-biological means, using of hemodialysis and/or plasmapheresis for detoxifying the patient's blood through action of adsorption, filtration or centrifugation separation of the blood elements. Such approaches of developing non-biological liver support therapies have, however, yielded only limited or no success. The development of bio-artificial liver devices capable of supporting multiple liver specific functions began to gather speed as advances made in liver biology revealed the liver as a complex organ supporting multiple synthetic and metabolic functions. It thus became clear that replacement of a single hepatic function such as blood detoxification is not sufficient. Recent years have accordingly witnessed the advent of the bioartificial liver assist devices (BLADs) as a promising therapeutic for the treatment of acute liver failure. These BLA-type devices include mammalian cells and serve as an extracorporeal support for liver failure patients. Unlike a membrane-based liver dialysis system that provides passive detoxification treatment for patients with liver failure, BLADs set out to take over most if not all of the liver functions in the patient. This is achieved through the integration of isolated hepatocytes (liver cells) either of allogeneic or xenogeneic origin (e.g. porcine hepatocytes) with the ex vivo perfusion membrane-based bioreactors and perfused with either patients's plasma or blood. By using hepatocytes the BLAD not only performs blood detoxification but also provides liver-synthetic functions. Accordingly, the basic design consideration for BLADs is to ensure the maintenance of cell viability and functions with optimal control of nutrient and gases, coupled with metabolites exchanges, within the bioreactor cell culture.
Currently bio-artificial liver support systems of several alternative designs are either under development or undergoing pre-clinical/clinical trials. These BLADs can be classified into four basic designs according to the method of cells immobilization and the mode of perfusion used: 1) semi-permeable hollow fiber membranes, 2) encapsulations, 3) flatbed, and 4) perfused bed or scaffolds. In these configurations, the cells can be immobilized by Means of a micro-carrier or be seeded in a collagen gel matrix or polymeric scaffold to form a 2D or 3D culture. Among the numerous BLAD designs, the hollow-fiber based liver support system, has been the most intensively studied and reviewed. Hollow fiber based BLAD configurations provide the ease of scaling up and the cells embedded within the hollow fiber are protected from the direct effect of the fluid shear as a result of the semi-permeable hollow fiber membrane. Certain difficulties nevertheless remain with this architecture, such as achieving uniform cell distribution within the luminal or capillary spaces of the hollow fiber and an inadequacy of the oxygen supply to the cell surfaces. The latter problem leads to the buildup of the non-physiological gradients in the bioreactor. In addition, hepatocytes are polarized cells and the liver specific functions are highly depends on cell polarity. During isolation, hepatocytes quickly lose their functions since cell polarity is disrupted. The lack of a cellular microenvironment to re-establish hepatocyte polarity in these configurations results in the deterioration of hepatocyte in vitro functions, thereby compromising the effectiveness of these BLADs.
An alternative architecture used is the flatbed bioreactor, mostly based on monolayer or sandwich culture. Such a design generally provides better control in terms of the uniformity of cell distribution and the regulation of micro environments mimicry of that found in vivo. Hepatocytes seeded as a monolayer on a substrate such as a collage gel form good cell-cell interaction as well as cell-matrix interaction. In the sandwich culture, the overlay of another collage gel layer on the top of the monolayer further stabilizes the structure. Hepatoycte polarity is re-established in sandwich culture, characterized by the exhibition of tight and gap junction, the formation of bile canalicular network, and the localization of sinusoidal and biliary transporters on the cell membrane, so that in vitro hepatocyte functions can be long-term maintained. Difficulties arising with this flatbed architecture include dead volume, regulation of the shear stress level on the cell surfaces, scalability of the bioreactor or low surface area to volume ratio. These problems have not been adequately addressed in many existing designs.
Thus on a general basis, the major drawback for many of the BLADs currently undergoing preclinical and clinical trials is the long-term maintenance of cell viability and functions as a result of inadequate cellular microenvironment and bidirectional mass transport. Indeed, research has shown that these bioreactors suffer from the rapid decline of cell functions due to poor maintenance of microenvironment cues, the inefficiency of nutrients and gas transport, and the inadequacy of the removal of accumulated toxic metabolites from the cells.
Accordingly it is an object of the present invention to provide a device or apparatus that overcomes at least some of these drawbacks. This object is solved by providing an apparatus according to claim 1.